CSS flex

The CSS flex property is a shorthand property for setting a flexible length on flex items. Specifically, it sets the flex-grow, flex-shrink, and flex-basis properties.

Flex items are placed within a flex container. A flex container is an element with either display: flex or display: inline-flex.

In the flex layout model, the children of a flex container can be laid out in any direction, and can "flex" their sizes, either growing to fill unused space or shrinking to avoid overflowing the parent.

The W3C encourages authors to use this flex shorthand property rather than its longhand properties directly, as the shorthand correctly resets any unspecified components to accommodate common uses.

Syntax

Possible Values

<'flex-grow'>

Sets the flex-grow longhand property. The value specifies how much the flex item will grow relative to the rest of the flex items in the flex container when positive free space is distributed. Value must be a number.

<'flex-shrink'>

Sets the flex-shrink longhand property. The value specifies how much the flex item will shrink relative to the rest of the flex items in the flex container when negative free space is distributed. Value must be a number.

<'flex-basis'>

Sets the flex-basis longhand property. The value specifies the initial main size of the flex item, before free space is distributed according to the flex factors. This property takes the same values as the width property (except auto is treated differently), as well as an additional content keyword. You can omit this value from the flex shorthand property, which will result in a value of 0%.

If the specified value is auto, the flex basis is the computed value of the flex item's main size property. If that value is itself auto, then the flex basis is automatically-determined based on its content (i.e. sized as for content).

In addition, all CSS properties also accept the following CSS-wide keyword values as the sole component of their property value:

initial

Represents the value specified as the property's initial value.

inherit

Represents the computed value of the property on the element's parent.

unset

This value acts as either inherit or initial, depending on whether the property is inherited or not. In other words, it sets all properties to their parent value if they are inheritable or to their initial value if not inheritable.

Example Code

Basic CSS

Working Example within an HTML Document

CSS Specifications

Browser Support

The following table provided by Caniuse.com shows the level of browser support for this feature.

Vendor Prefixes

For maximum browser compatibility many web developers add browser-specific properties by using extensions such as -webkit- for Safari, Google Chrome, and Opera (newer versions), -ms- for Internet Explorer, -moz- for Firefox, -o- for older versions of Opera etc. As with any CSS property, if a browser doesn't support a proprietary extension, it will simply ignore it.

This practice is not recommended by the W3C, however in many cases, the only way you can test a property is to include the CSS extension that is compatible with your browser.

The major browser manufacturers generally strive to adhere to the W3C specifications, and when they support a non-prefixed property, they typically remove the prefixed version. Also, W3C advises vendors to remove their prefixes for properties that reach Candidate Recommendation status.

Many developers use Autoprefixer, which is a postprocessor for CSS. Autoprefixer automatically adds vendor prefixes to your CSS so that you don't need to. It also removes old, unnecessary prefixes from your CSS.

You can also use Autoprefixer with preprocessors such as Less and Sass.